Attention is directed to my co-pending patent application entitled "Process for Selectively Etching Silicon" filed May 25, 1982, herein incorporated by reference.
Aluminum and alloys of aluminum with silicon and/or copper are widely used in integrated circuit metallization, and consequently a great deal of effort has been expended on dry etch processes which will enable fine Al patterns to be formed. The general approach which has been adopted is reactive sputter etching using an etch gas that reacts with Al to form a volatile reaction product when it is excited by an rf discharge.
For an example of the reactive sputter etching technique, see U.S. Pat. No. 3,994,793 issued to Harvilchuck on Nov. 30, 1976. Unfortunately, the only easily volatile Al compound is AlCl.sub.3, and the requisite Cl in the discharge is exceptionally reactive with bare (unoxidized) Al. This reactivity results in loading effects and in lateral etching under the mask. In addition, the AlCl.sub.3 residues are hygroscopic and form HCl upon air exposure which further attacks the metal. In contrast the native Al.sub.2 O.sub.3 oxide layer is typically etched very slowly in Cl discharges, leading to long etch "initiation" times and to grain boundary images which propagate into the substrate.
Reactive sputter etching of Al thus has problems which are mainly related to the high reactivity of Cl.sub.2 with Al. One remedy which has been proposed is to etch the Al patterns using nonreactive sputter etching or ion milling. However, these processes redeposit the etched metal on mask sidewalls, and have poor etch rate selectivity between materials. In addition, mask profiles are altered by "faceting", and substrates are anomalously etched near metal lines ("trenching").
It should be noted that a variety of ionic species have been used in reactive sputter etching of semiconductor materials (as opposed to aluminum or aluminum alloys). Considerable efforts have been made to develop selective etching techniques for semiconductors that can, for example, etch silicon dioxide more rapidly than silicon or, conversely, remove silicon from a surface faster than SiO.sub.2. See, for example, U.S. Pat. No. 4,213,813 issued to Lemons on July 22, 1980, wherein additives such as O.sub.2 are proposed to achieve selective etching. I am not aware of any prior teachings of the use of such additives in connection with aluminum etching.
There exists a need for selective etching techniques for aluminum that would permit the formation of fine Al patterns. Additionally there exists a need for new etchants that are more controllable and less corrosive than chlorine-based aluminum etches.